Ace should win. I wonder if HT could burn Ace the same way Akainu do, after all he is hotter. But then again, maybe thats not enough, maybe you need haki. And, Ace is far stronger and faster in a fight, and he has Haōshoku Haki, so he could just KO him.

Does not merely ignite himself with flame but is flame itself Is strong even without his fire powers Has an additional power unrelated to fire, haki

Ace is fire itself and can't be harmed by other fire. Torch still has a human body but as he can generate temperatures even higher than Ace, so he should not be harmed by Ace's flames either. Strictly fire vs fire and it is a complete stalemate. The deciding factor here is Ace's additional power haki.

@Suiken_Seiji: You can extinguish flames with another. It's easy. Fire draws in oxygen and requires oxygen to work. HT just have to drain the oxygen from the air.Something he can very easily in his Nova Form. Nova Form runs at the constant risk of igniting the atmosphere and ridding the planet from life. That's why HT refrains from using that unless its necessary. Moreals off or fight on an uninhabitated planet and he would casually do that. No air=no flames for Ace.